Onalaska is a city in La Crosse County, Wisconsin, United States. The population was 18,803 at the 2020 census. It borders the larger La Crosse, Wisconsin, and is a part of the La Crosse-Onalaska, WI-MN Metropolitan Area. Onalaska is built on a slightly elevated ridge above the Black River. Natural areas include both river bottom land and high, heavily wooded, scenic bluffs. A man-made reservoir at the city's western edge is known as Lake Onalaska.